How to check memory usage in Linux

In the Linux operating system it is important to have enough free physical memory (RAM) available for your processes which need to run. If this memory runs out, swap space will start to be used next, which will result in slower performance as I/O operations will need to be written to disk instead. If both main memory and swap become full your Linux system can completely freeze up.

By checking and monitoring the amount of memory in use and confirming there is enough free memory to service requests we can ensure that processes continue to run optimally.

Commands to check Linux memory usage

  1. The ‘free’ command.
  2. The ‘free’ command will provide the most accurate way of showing memory use, when run with the -m flag the output is easier to read as values will be shown in MB.

    root@server1 [~]# free -m
                 total       used       free     shared    buffers     cached
    Mem:          3948       3248        700          0        245       2036
    -/+ buffers/cache:        966       2982
    Swap:         3999        675       3324

    If you see that the amount of used Mem is high and free Mem is low, don’t panic (at least not straight away) as there is usually memory held for caching purposes to help speed things up. The best indication of unused memory is the free column for the buffers/cache row. This displays how much memory is completely unused and available, as the memory used by buffers/cache can quickly be used elsewhere should it be required.

    Linux is smart enough to take some of your unused memory for disk caching to increase performance, however it can make it appear as if you have less freely available memory than you actually do. It’s technically in use by the operating system which is why it shows as in use, however should you actually require any of this memory elsewhere it will be ready to use straight away, so you may not necessarily be low on memory.

    In the above example, the used Mem shows as 3248mb with 700mb free however this is not the best indication of free memory. The best would be the 2982mb listed free in buffers/cache.

    The free command will also show your used and free swap usage, ideally 0 swap will be used, meaning that nothing has been swapped out from physical memory to hard disk. If this value is higher than 0, you should investigate memory usage as it may have filled up and spilled over to disk at some point. In our above example we have some minor swap usage which has not changed in some time and there is plenty of free memory, so currently it looks alright.

    If swap fills up it is possible to assign more disk space to increase it, however first you would be better off confirming the additional memory usage is required, and then you would want to increase physical memory.

  3. The ‘top’ command.
  4. The top command is a well known way of getting a fast overview of resource usage in Linux dynamically in real time. Top shows us both the total memory available including how much of it is in use and how much is free. The top command also shows us how much swap space is available including how much is in use and free. These values include memory used by buffers and cache, similar to the first line shown in the ‘free’ command.

    top - 22:39:56 up 7 days,  9:58,  1 user,  load average: 0.03, 0.05, 0.00
    Tasks: 124 total,   1 running, 123 sleeping,   0 stopped,   0 zombie
    Cpu(s):  0.2%us,  0.0%sy,  0.0%ni, 99.8%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
    Mem:   4043720k total,  3325664k used,   718056k free,   251452k buffers
    Swap:  4095992k total,   691520k used,  3404472k free,  2085212k cached
     5034 mysql     15   0 1443m 207m 6552 S  0.0  5.3  20:07.09 mysqld
     8320 username  16   0  149m  46m 6848 S  0.0  1.2   0:11.94 php
     8631 username  16   0  148m  45m 6832 S  0.0  1.2   0:21.39 php
     8269 username  16   0  146m  43m 7252 S  0.0  1.1   0:14.85 php
     8694 username  15   0  145m  43m 7272 S  0.0  1.1   0:12.74 php
     8695 username  16   0  145m  42m 7276 S  0.0  1.1   0:12.60 php
     9110 username  16   0  142m  40m 7248 S  0.0  1.0   0:03.30 php
     8604 username  16   0  140m  37m 7192 S  0.0  1.0   0:03.86 php
     8603 username  15   0  138m  35m 7204 S  0.0  0.9   0:05.54 php
     8289 username  16   0  184m  29m 6648 S  0.0  0.7   0:06.77 php
     2158 root      11 -10 28704  22m 1744 S  0.0  0.6   0:00.01 iscsiuio
     8768 username  16   0  179m  20m 6680 S  0.0  0.5   0:01.44 php
     8290 username  16   0  175m  17m 6336 S  0.0  0.5   0:14.69 php
     8106 nobody    20   0  416m  12m 7316 S  0.0  0.3   0:00.97 httpd
     8113 nobody    19   0  351m  10m 6832 S  0.0  0.3   0:00.08 httpd

    We can then go a step further and sort the processes in top by amount of memory in use, allowing us to rank the processes from highest memory use to lowest. This will help us see if there are any memory heavy processes that may need to be investigated further. To sort the top output by memory, press the “shift + m” keys.

    The output in the above example has been sorted this way, we can see that the mysqld process is using the most amount of memory, which makes sense as it has been configured to cache databases.

  5. The /proc/meminfo file
  6. If you’re after an even more detailed break down on your memory, take a look at the /proc/meminfo file.

    root@server1 [~]# cat /proc/meminfo
    MemTotal:      4043720 kB
    MemFree:        715944 kB
    Buffers:        251940 kB
    Cached:        2085324 kB
    SwapCached:     206052 kB
    Active:        1862372 kB
    Inactive:      1141632 kB
    HighTotal:           0 kB
    HighFree:            0 kB
    LowTotal:      4043720 kB
    LowFree:        715944 kB
    SwapTotal:     4095992 kB
    SwapFree:      3404472 kB
    Dirty:             740 kB
    Writeback:           0 kB
    AnonPages:      626324 kB
    Mapped:          34684 kB
    Slab:           284540 kB
    PageTables:      15068 kB
    NFS_Unstable:        0 kB
    Bounce:              0 kB
    CommitLimit:   6117852 kB
    Committed_AS:  3057500 kB
    VmallocTotal: 34359738367 kB
    VmallocUsed:    265844 kB
    VmallocChunk: 34359471351 kB
    HugePages_Total:     0
    HugePages_Free:      0
    HugePages_Rsvd:      0
    Hugepagesize:     2048 kB


We have covered a few tools you can use to see how much physical memory (RAM) is in use in Linux. The free command will show memory in use and free memory for the whole system including memory used by buffers and cache, while the top command will show real time usage information of memory broken down by specific processes allowing you to see how much memory each process is using.

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